Process 1 is the sudden collapse of the wave function from a superposition of quantum states into a single state, with the probability of collapsing into a given state proportional to the overlap of the wave functions of new state with each of the superposition states. (See von Neumann Process 1.)

Process 2 is the unitary time evolution of the wave function deterministically generated by the Schrödinger wave equation. (See von Neumann Process 2.)

Everett then presents the internal contradictions of observer-dependent collapses of wave functions with examples of "Wigner's Friend," an observer who observes another observer. For whom does the wave function collapse?

Everett considers several alternative explanations for Wigner's paradox, the fourth of which is the standard statistical interpretation of quantum mechanics, which was criticized by Einstein as not being a complete description.

Alternative 4: To abandon the position that the state function is a
complete description of a system. The state function is to be regarded
not as a description of a single system, but of an ensemble
of systems, so that the probabilistic assertions arise naturally
from the incompleteness of the description.

("The Many-Worlds Interpretation of Quantum Mechanics," p.8)

In order to be "complete, Everett thought that "hidden variables" would be necessary.

Everett's "theory of the universal wave function" is the last alternative:

Alternative 5: To assume the universal validity of the quantum description,
by the complete abandonment of Process 1. The general
validity of pure wave mechanics, without any statistical assertions,
is assumed for all physical systems, including observers and measuring
apparata. Observation processes are to be described completely
by the state function of the composite system which includes
the observer and his object-system, and which at all times
obeys the wave equation (Process 2).

("The Many-Worlds Interpretation of Quantum Mechanics," p.8)

He says this alternative has many advantages.

It has logical simplicity and it is complete in the sense that it is
applicable to the entire universe. All processes are considered equally
(there are no "measurement processes" which play any preferred role),
and the principle of psycho-physical parallelism is fully maintained. Since
the universal validity of the state function description is asserted, one
can regard the state functions themselves as the fundamental entities,
and one can even consider the state function of the whole universe. In
this sense this theory can be called the theory of the "universal wave
function, " since all of physics is presumed to follow from this function.

("The Many-Worlds Interpretation of Quantum Mechanics," p.8)

Information and Entropy

In a lengthy chapter, Everett develops the concept of information - despite the fact that his deterministic view of physics allows no possibilities. For Claude Shannon, the developer of the theory of communication of information, there can be no information transmitted without possibilities. Everett correctly observes that in classical mechanics information is a conserved property, a constant of the motion. No new information can be created in the universe.

As a second illustrative example we consider briefly the classical
mechanics of a group of particles. The system at any instant is represented
by a point...in the phase
space of all position and momentum coordinates. The natural motion of
the system then carries each point into another, defining a continuous
transformation of the phase space into itself. According to Liouville's
theorem the measure of a set of points of the phase space is invariant
under this transformation. This invariance of measure implies that if we
begin with a probability distribution over the phase space, rather than a
single point, the total information,...
which is the information of the joint distribution for all positions and
momenta, remains constant in time.

("The Many-Worlds Interpretation of Quantum Mechanics," p.31)

Everett correctly notes that if total information is constant, the total entropy is also constant.

if one were to
define the total entropy to be the negative of the total information, one
could replace the usual second law of thermodynamics by a law of
conservation of total entropy, where the increase in the standard (marginal)
entropy is exactly compensated by a (negative) correlation entropy. The
usual second law then results simply from our renunciation of all correlation
knowledge (stosszahlansatz), and not from any intrinsic behavior of
classical systems. The situation for classical mechanics is thus in sharp
contrast to that of stochastic processes, which are intrinsically irreversible.

("The Many-Worlds Interpretation of Quantum Mechanics," p.31-32)

The Appearance of Irreversibility in a Measurement

There is another way of looking at this apparent irreversibility within
our theory which recognizes only Process 2. When an observer performs
an observation the result is a superposition, each element of which describes
an observer who has perceived a particular value. From this time
forward there is no interaction between the separate elements of the superposition
(which describe the observer as having perceived different results),
since each element separately continues to obey the wave equation. Each
observer described by a particular element of the superposition behaves
in the future completely independently of any events in the remaining elements,
and he can no longer obtain any information whatsoever concerning
these other elements (they are completely unobservable to him).

The irreversibility of the measuring process is therefore, within our
framework, simply a subjective manifestation reflecting the fact that in
observation processes the state of the observer is transformed into a
superposition of observer states, each element of which describes an observer
who is irrevocably cut off from the remaining elements. While it is
conceivable that some outside agency could reverse the total wave function,
such a change cannot be brought about by any observer which is
represented by a single element of a superposition, since he is entirely
powerless to have any influence on any other elements.

There are, therefore, fundamental restrictions to the knowledge that
an observer can obtain about the state of the universe. It is impossible
for any observer to discover the total state function of any physical system,
since the process of observation itself leaves no independent state
for the system or the observer, but only a composite system state in which
the object-system states are inextricably bound up with the observer states.

Here is Everett's radical thesis that the observation "splits" the single observer into a superposition of multiple observers, each one of which has knowledge only of the new object-system state (interpreted later by Bryce DeWitt as different "universes")

As soon as the observation is performed, the composite state is split into
a superposition for which each element describes a different object-system
state and an observer with (different) knowledge of it. Only the totality
of these observer states, with their diverse knowledge, contains complete
information about the original object-system state - but there is no possible
communication between the observers described by these separate states.
Any single observer can therefore possess knowledge only of the
relative state function (relative to his state) of any systems, which is in
any case all that is of any importance to him.

("The Many-Worlds Interpretation of Quantum Mechanics," pp.97-98)

In the final chapter of his thesis, Everett gives five possible "interpretations, the "popular", the "Copenhagen", the "hidden variables", the "stochastic process", and the "wave" interpretations.

a. The "popular" interpretation. This is the scheme alluded to in
the introduction, where ψ is regarded as objectively characterizing
the single system, obeying a deterministic wave equation when
the system is isolated but changing probabilistically and discontinuously
under observation.

("The Many-Worlds Interpretation of Quantum Mechanics," p.110)

b. The Copenhagen interpretation. This is the interpretation developed by Bohr. The ψ function is not regarded as an objective description
of a physical system (i.e., it is in no sense a conceptual
model), but is regarded as merely a mathematical artifice which
enables one to make statistical predictions, albeit the best predictions
which it is possible to make. This interpretation in fact
denies the very possibility of a single conceptual model applicable
to the quantum realm, and asserts that the totality of phenomena
can only be understood by the use of different, mutually exclusive
(i.e., "complementary") models in different situations. All statements about microscopic phenomena are regarded as meaningless
unless accompanied by a complete description (classical) of an
experimental arrangement.

("The Many-Worlds Interpretation of Quantum Mechanics," p.110)

c. The "hidden variables" interpretation. This is the position
(Alternative 4 of the Introduction) that ψ is not a complete description
of a single system. It is assumed that the correct complete
description, which would involve further (hidden) parameters,
would lead to a deterministic theory, from which the probabilistic
aspects arise as a result of our ignorance of these extra parameters
in the same manner as in classical statistical mechanics.

("The Many-Worlds Interpretation of Quantum Mechanics," p.111)

Everett says that here the ψ-function is regarded as a description of an ensemble
of systems rather than a single system. Proponents of this interpretation
include Einstein and Bohm.

The stochastic process interpretation. This is the point of view
which holds that the fundamental processes of nature are stochastic
(i.e., probabilistic) processes. According to this picture
physical systems are supposed to exist at all times in definite
states, but the states are continually undergoing probabilistic
changes. The discontinuous probabilistic "quantum-jumps" are
not associated with acts of observation, but are fundamental to the
systems themselves.

The wave interpretation. This is the position proposed in the
present thesis, in which the wave function itself is held to be the
fundamental entity, obeying at all times a deterministic wave
equation.

Everett proposed that the complicated problem of "conscious observers" can be greatly simplified by noting that the most important element in an observation is the recorded information about the measurement outcome in the memory of the observer. He proposed that human observers could be replaced by automatic measurement equipment that would achieve the same result. A measurement would occur when information is recorded by the measuring instrument.

It will suffice for our purposes to consider the observers
to possess memories (i.e., parts of a relatively
permanent nature whose states are in correspondence
with past experience of the observers). In order to
make deductions about the past experience of an observer
it is sufficient to deduce the present contents of
the memory as it appears within the mathematical
model.

As models for observers we can, if we wish, consider
automatically functioning machines, possessing sensory
apparatus and coupled to recording devices capable of
registering past sensory data and machine configurations.

Everett's observer model is a classic example of artificial intelligence.

We can further suppose that the machine is so
constructed that its present actions shall be determined
not only by its present sensory data, but by the contents
of its memory as well. Such a machine will then
be capable of performing a sequence of observations
(measurements), and furthermore of deciding upon its
future experiments on the basis of past results. If we
consider that current sensory data, as well as machine
configuration, is immediately recorded in the memory,
then the actions of the machine at a given instant can
be regarded as a function of the memory contents only,
and all relevant experience of the machine is contained
in the memory.

Everett's observer model has what might be called artificial consciousness.

For such machines we are justified in using such
phrases as "the machine has perceived A" or "the
machine is aware of A" if the occurrence of A is represented
in the memory, since the future behavior of
the machine will be based upon the occurrence of A. In
fact, all of the customary language of subjective experience
is quite applicable to such machines, and forms the
most natural and useful mode of expression when
dealing with their behavior, as is well known to individuals
who work with complex automata.

Everett's model of machine memory completely solves the problem of "Wigner's Friend." As in the information interpretation of quantum mechanics, it is the recording of information in a "measurement" that makes a subsequent "observation" by a human observer possible.

Summary of Everett's Ideas

Everett's idea for the "universal validity of the quantum description" can be read as saying that quantum mechanics applies to all physical systems, not merely microscopic systems. Then "classical" mechanics emerges in the limit of the Planck quantum of action h → 0, or more importantly, h / m → 0, so that classical physics appears in large massive objects (like human beings) because the indeterminacy is too small to measure.

Everett says that the ψ-function is a description of an ensemble of systems rather than a single system. It is true that the phenomenon of wave interference is only inferred from the results of many single particle experiments. We do not "see" interference directly. Probabilistic assertions arise naturally from the incompleteness of the description.

Everett correctly observes that in classical mechanics information is a conserved property, a constant of the motion. No new information can be created in a classical universe.

The Everett theory preserves the "appearance" of possibilities as well as all the results of standard quantum mechanics. It is an "interpretation" after all. So even wave functions "appear" to collapse. Note that if there are many possibilities, whenever one becomes actual, the others disappear instantly in standard quantum physics. In Everett's theory, they become other possible worlds